At the very simplest, you can just overlap things in the slicer without Blender.
If you want to learn about Blender’s Sculpt mode, you can just Google “Blender Sculpt mode tutorial”. For convenience, try to use the most recent results, as the interface can be slightly different in older versions.
Sculpt mode effectively allows you to alter the models as if they were made of clay or plasticine.
A lot of the tutorials will be showing how to make things from scratch, but what’s important is that you see how the tools work.
Once you have everything overlapping the way you want, you can join the using a Boolean operation. You’ll want to use a “union” operation.
Overhangs were the biggest issue I found. So much so that I moved back to a 0.4 after a month of faff trying to find settings that would compensate.
I use my printer mainly for minis, and figures that would print supportless on the 0.4 nozzle needed huge amounts of supports at 0.2 in order to print without missing chins etc.
That said, the level of detail that I could achieve was better, particularly on the hair, but not enough to compensate for all of the extra faff and wasted plastic.